Tom H. Johnston

The Pharmacology of l-DOPA-Induced Dyskinesia in Parkinson’s Disease

l-3,4-Dihydroxyphenylalanine (l-DOPA) remains the most effective symptomatic treatment of Parkinson’s disease (PD). However, long-term administration of l-DOPA is marred by the emergence of abnormal involuntary movements, i.e., l-DOPA-induced dyskinesia (LID). Years of intensive research have yielded significant progress in the quest to elucidate the mechanisms leading to the development and expression of dyskinesia and maintenance of the dyskinetic state, but the search for a complete understanding is still ongoing. Herein, we summarize the current knowledge of the pharmacology of LID in PD. Specifically, we review evidence gathered from postmortem and pharmacological studies, both preclinical and clinical, and discuss the involvement of dopaminergic and nondopaminergic systems, including glutamatergic, opioid, serotonergic, γ-aminobutyric acid (GABA)-ergic, adenosine, cannabinoid, adrenergic, histaminergic, and cholinergic systems. Moreover, we discuss changes occurring in transcription factors, intracellular signaling, and gene expression in the dyskinetic phenotype. Inasmuch as a multitude of neurotransmitters and receptors play a role in the etiology of dyskinesia, we propose that to optimally alleviate this motor complication, it may be necessary to develop combined treatment approaches that will target simultaneously more than one neurotransmitter system. This could be achieved via three ways as follows: 1) by developing compounds that will interact simultaneously to a multitude of receptors with the required agonist/antagonist effect at each target, 2) by targeting intracellular signaling cascades where the signals mediated by multiple receptors converge, and/or 3) to regulate gene expression in a manner that has effects on signaling by multiple pathways

Reduction of l-DOPA-Induced Dyskinesia by the Selective Metabotropic Glutamate Receptor 5 Antagonist 3-[(2-Methyl-1,3-thiazol-4-yl)ethynyl]pyridine in the 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-Lesioned Macaque Model of Parkinson's Disease

Long-term motor complications of dopamine replacement, such as l-DOPA-induced dyskinesia (LID) and reduced quality of l-DOPA action, remain obstacles in the treatment of Parkinsons disease. Dysfunctional glutamatergic neurotransmitter systems have been observed in both the untreated parkinsonian and dyskinetic states and represent novel targets for treatment. Here, we assess the pharmacokinetic profile and corresponding pharmacodynamic effects on behavior of the orally active, selective metabotropic glutamate receptor type 5 (mGlu5) antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) (as the hydrochloride salt) in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned macaque. Six parkinsonian, MPTP-lesioned cynomolgus monkeys, with established LID, were administered acute challenges with MTEP (4.5–36 mg/kg p.o.) or vehicle, either alone or in combination with l-DOPA (33 ± 1 mg/kg p.o.). Motor activity, parkinsonian disability, and dyskinesia were assessed for a 6-h period. Plasma drug levels were assessed by liquid chromatography-tandem mass spectrometry. MTEP had no antiparkinsonian action as monotherapy. However, administration of l-DOPA in combination with MTEP (36 mg/kg) reduced peak dose LID by 96%. Moreover, although total on-time (duration for which l-DOPA exerted an antiparkinsonian effect) was not significantly reduced, MTEP (36 mg/kg) reduced the duration of on-time with disabling LID by 70% compared with that for l-DOPA alone. These effects were associated with a peak plasma concentration of 20.9 μM and an area under the curve from 0 to 24 h of 136.1 h · μM (36 mg/kg). Although total on-time was not reduced, the peak antiparkinsonian benefit of l-DOPA/MTEP (36 mg/kg) was less than that with l-DOPA alone. Selective mGlu5 inhibitors may have significant potential to ameliorate dyskinesia, but care should be taken to ensure that such effects do not come at the expense of the peak antiparkinsonian benefit of l-DOPA.

Dopamine D3 receptor stimulation underlies the development of L-DOPA-induced dyskinesia in animal models of Parkinson's disease.

Development of L-DOPA-induced dyskinesia (LID) remains a major problem in the long-term treatment of Parkinsons disease (PD). Sensitization to L-DOPA correlates with ectopic expression of D3 dopamine receptors in the striatum, implicating D3 receptors in development of LID. We demonstrate that the selective D3 antagonist S33084 abolishes development of LID over 30 days in MPTP-lesioned marmosets without effecting the anti-parkinsonian actions of L-DOPA. Furthermore, following a 14 day washout, when challenged with L-DOPA in the absence of S33084, these animals continued to exhibit reduced LID. In the 6-OHDA-lesioned rat, S33084 similarly attenuated development of behavioural sensitization to L-DOPA. Additionally, L-DOPA-induced elevations in striatal pre-proenkephalin-A (PPE-A) (but not PPE-B, phospho[Thr(34)]DARPP-32, D1, and D2 receptor mRNA or D3 receptor levels) were reduced in S33084 treated animals. Our data suggest a role for D3 receptors in the development of LID and suggest that initiating L-DOPA treatment with a D3 antagonist may reduce the development of LID in PD.

Increased 5-HT2A receptors in the temporal cortex of parkinsonian patients with visual hallucinations†

Well‐formed visual hallucinations (VH) are common in patients with Parkinsons disease (PD). The pathophysiology of VH in PD is unknown but may involve structures mediating visual processing such as the inferior temporal cortex. Serotonergic type 2A (5‐HT2A) receptors have been linked to many psychiatric disorders, including psychosis. We hypothesized that enhanced 5‐HT2A receptor levels may be involved in VH in PD. Autoradiographic binding using [3H]‐ketanserin and spiperone, to define 5‐HT2A receptors, was performed in 6 PD patients with VH, 6 PD patients without VH, and 5 healthy, age‐matched controls. The cerebral regions studied included the orbitofrontal cortex, inferolateral temporal cortex, motor cortex, striatum, and substantia nigra. There was a significant (45.6%) increase in the levels of [3H]‐ketanserin binding in the inferolateral temporal cortex of PD patients with VH when compared with PD patients without VH (54.3 ± 5.2 fmol/mg vs. 37.3 ± 4.3 fmol/mg, P = 0.039). Additionally, there was a significant increase in the levels of 5‐HT2A receptors in the motor cortex of all PD patients taken as a group when compared with controls (57.8 ± 5.7 fmol/mg vs. 41.2 ± 2.6 fmol/mg, P = 0.0297). These results suggest that enhanced 5‐HT2A‐mediated neurotransmission in the inferolateral temporal cortex, a critical structure in visual processing, might be associated with the development of VH in PD. Our results provide new insights into the pathophysiology of VH in PD and provide an anatomical basis to explain why compounds with 5‐HT2A antagonist activity are effective at alleviating this debilitating complication.

Pharmacological characterization of psychosis-like behavior in the MPTP-lesioned nonhuman primate model of Parkinson's disease

Investigation of the pathophysiology of psychosis in Parkinsons disease (PD), as well as the assessment of potential novel therapeutics, has been limited by the lack of a well‐validated animal model. MPTP‐lesioned primates exhibit abnormal behaviors that are distinct from dyskinesia and parkinsonism and may represent behavioral correlates of neural processes related to psychosis in PD. Here we assess four types of behavior—agitation, hallucinatory‐like responses to nonapparent stimuli, obsessive grooming, and stereotypies that are termed “psychosis‐like”—and define their pharmacology using a psychosis‐like behavior rating scale. By assessing the actions of drugs known to enhance or attenuate psychosis in PD patients, we find that the pharmacology of these behaviors recapitulates, in several respects, the pharmacology of psychosis in PD. Thus, levodopa and apomorphine elicited psychosis‐like behaviors. Amantadine significantly decreased levodopa‐induced dyskinesia but exacerbated psychosis‐like behaviors. Haloperidol reduced psychosis‐like behaviors but at the expense of increased parkinsonian disability while the atypical neuroleptics clozapine and quetiapine reduced psychosis‐like behaviors without significant effect on parkinsonian disability. The response of different components of the psychotomimetic behavior suggested the involvement of both dopaminergic and nondopaminergic mechanisms in their expression.

PYM50028, a novel, orally active, nonpeptide neurotrophic factor inducer, prevents and reverses neuronal damage induced by MPP+ in mesencephalic neurons and by MPTP in a mouse model of Parkinson’s disease

Many experimental data support the enhancement of neurotrophic factors as a means to modify neurodegeneration in Parkinsons disease. However, the translation of this to the clinic has proven problematic. This is likely due to the complex nature of the surgical gene delivery and cell‐based approaches adopted to deliver proteinaceous neurotrophic factors to targets within the central nervous system. We investigated the ability of a novel, orally active, nonpeptide neurotrophic factor inducer, PYM50028 (Cogane), to restore dopaminergic function after 1‐methyl‐4‐phenylpyridinium (MPP+) ‐induced damage to mesencephalic neurons in vitro and in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP) ‐lesioned mice. In rat mesencephalic neurons, administration of PYM50028, either before or after MPP+, significantly prevented and reversed both MPP+‐induced neuronal atrophy and cell loss. These effects were potent and of a magnitude equivalent to those achieved by a combination of brain‐derived neurotrophic factor (BDNF) and glial‐derived neurotrophic factor (GDNF). Oral admin istration of PYM50028 (10 mg/kg/day for 60 days) to MPTP‐lesioned mice, commencing after a striatal impairment was evident, resulted in a significant elevation of striatal GDNF (297%) and BDNF (511%), and attenuated the loss of striatal dopaminergic transporter levels and dopaminergic neurons in the substantia nigra. PYM50028 did not inhibit monoamine oxidase B in vitro, nor did it alter brain levels of MPP+ in vivo. PYM50028 has neuroprotective and neurorestorative potential and is in clinical development for the treat ment of neurodegenerative disorders, including Parkinsons disease.—Visanji, N. P., Orsi, A., Johnston, T. H., Howson, P. A., Dixon, K., Callizot, N., Brotchie, J. M., Rees, D. D. PYM50028, a novel, orally active, nonpeptide neurotrophic factor inducer, prevents and reverses neuronal damage induced by MPP+ in mesencephalic neurons and by MPTP in a mouse model of Parkinsons disease. FASEB J. 22, 2488–2497 (2008)

Histamine H3 receptor agonists reduce L‐dopa–induced chorea, but not dystonia, in the MPTP‐lesioned nonhuman primate model of Parkinson's disease

L‐dopa–induced dyskinesia (LID) remains a major complication of the treatment of Parkinsons disease. The neural mechanisms underlying LID are thought to involve overactivity of striatal glutamatergic neurotransmission, with resultant underactivation of the output regions of the basal ganglia. Histamine H3 heteroreceptors can reduce glutamate and γ‐aminobutyric acid (GABA) transmission in the striatum and substantia nigra reticulata, respectively. Thus, we tested whether the histamine H3 receptor agonists immepip and imetit can alleviate LID in the MPTP‐lesioned marmoset model of Parkinsons disease. Coadministration of immepip (1 mg/kg) with L‐dopa (15 mg/kg) was associated with significantly less total dyskinesia than L‐dopa alone. When dyskinesia was separately rated as chorea and dystonia, coadministration of L‐dopa with either immepip or imetit (both 10 mg/kg) significantly reduced chorea but had no effect on dystonia. The antidyskinetic actions of the H3 agonists were not accompanied by alteration of the antiparkinsonian actions of L‐dopa. However, immepip (10 mg/kg), when administered as monotherapy, significantly increased parkinsonian disability compared to vehicle. Overall, the results obtained in this study suggest that histamine H3 receptors may be involved in the neural mechanisms underlying L‐dopa–induced dyskinesia in Parkinsons disease.

Expression of human A53T alpha-synuclein in the rat substantia nigra using a novel AAV1/2 vector produces a rapidly evolving pathology with protein aggregation, dystrophic neurite architecture and nigrostriatal degeneration with potential to model the pathology of Parkinson's disease

BackgroundThe pathological hallmarks of Parkinsons disease (PD) include the presence of alpha-synuclein (α-syn) rich Lewy bodies and neurites and the loss of dopaminergic (DA) neurons of the substantia nigra (SN). Animal models of PD based on viral vector-mediated over-expression of α-syn have been developed and show evidence of DA toxicity to varying degrees depending on the type of virus used, its concentration, and the serotype of vector employed. To date these models have been variable, difficult to reproduce, and slow in their evolution to achieve a desired phenotype, hindering their use as a model for testing novel therapeutics. To address these issues we have taken a novel vector in this context, that can be prepared in high titer and which possesses an ability to produce neuronally-directed expression, with expression dynamics optimised to provide a rapid rise in gene product expression. Thus, in the current study, we have used a high titer chimeric AAV1/2 vector, to express human A53T α-syn, an empty vector control (EV), or green fluorescent protein (GFP), the latter to control for the possibility that high levels of protein in themselves might contribute to damage.ResultsWe show that following a single 2 μl injection into the rat SN there is near complete coverage of the structure and expression of A53T α-syn or GFP appears throughout the striatum. Within 3 weeks of SN delivery of their respective vectors, aggregations of insoluble α-syn were observed in SN DA neurons. The numbers of DA neurons in the SN were significantly reduced by expression of A53T α-syn (52%), and to a lesser extent by GFP (24%), compared to EV controls (both P < 0.01). At the level of the striatum, AAV1/2-A53T α-syn injection produced dystrophic neurites and a significant reduction in tyrosine hydroxylase levels (by 53%, P < 0.01), this was not seen in the AAV1/2-GFP condition.ConclusionsIn the current implementation of the model, we recapitulate the primary pathological hallmarks of PD, although a proportion of the SN damage may relate to general protein overload and may not be specific for A53T α-syn. Future studies will thus be required to optimise the dose of AAV1/2 employed before fully characterizing this model. The dynamics of the evolution of the pathology however, provide advantages over current models with respect to providing an initial screen to assess efficacy of novel treatments that might prevent/reverse α-syn aggregation.

A critique of available scales and presentation of the non-human primate dyskinesia rating scale†‡

Levodopa‐induced dyskinesia (LID) is a major limitation of long‐term management of Parkinsons disease. The roadblocks that have hindered the development of new treatments for levodopa‐induced dyskinesia were discussed at a meeting organized by the Michael J. Fox Foundation for Parkinsons research (New York, NY, March 2011). Among these, the lack of consensus methodology and clinical applicability for eliciting and rating LID in 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)–treated monkeys was highlighted as a particular concern. Here we present an update on the practical use of rating scales for evaluating LID in MPTP‐lesioned primate models of PD, with a focus on macaques, and present specifics on the Non‐Human Primate Dyskinesia Rating Scale.

Progressive neurodegeneration or endogenous compensation in an animal model of Parkinson's disease produced by decreasing doses of alpha-synuclein.

The pathological hallmarks of Parkinsons disease (PD) are degeneration of dopamine (DA) neurons of the substantia nigra (SN) and the presence of alpha-synuclein (α-syn)-rich Lewy bodies in DA cells that remain. To model these aspects of the disease, we previously showed that high titer (5.1×10exp12 gp/ml) AAV1/2 driven expression of A53T α-syn in the SN of rats caused nigrostriatal pathology including a loss of DA neurons, but also with toxicity in the GFP control group. In the current study, we evaluate the effects of two lower titers by dilution of the vector (1∶3 [1.7×10exp12] and 1∶10 [5.1×10exp11]) to define a concentration that produced pathology specific for α-syn. In GFP and empty vector groups there were no behavioural or post-mortem changes at 3 or 6 weeks post-administration at either vector dose. Dilution of the AAV1/2 A53T α-syn (1∶3) produced significant paw use asymmetry, reductions in striatal tyrosine hydroxylase (TH), and increases in DA turnover at 3 weeks in the absence of overt pathology. By 6 weeks greater evidence of pathology was observed and included, reductions in SN DA neurons, striatal DA, TH and DA-transporter, along with a sustained behavioural deficit. In contrast, the 1∶10 AAV1/2 A53T α-syn treated animals showed normalization between 3 and 6 weeks in paw use asymmetry, reductions in striatal TH, and increased DA turnover. Progression of dopaminergic deficits using the 1∶3 titer of AAV1/2 A53Tα-syn provides a platform for evaluating treatments directed at preventing and/or reversing synucleinopathy. Use of the 1∶10 titer of AAV1/2 A53T α-syn provides an opportunity to study mechanisms of endogenous compensation. Furthermore, these data highlight the need to characterize the titer of vector being utilized, when using AAV to express pathogenic proteins and model disease process, to avoid producing non-specific effects.